1. Field of the Invention
The present invention relates to a propelling transmission structure for a working vehicle having change speed gearing.
2. Description of the Related Art
One example of propelling transmission structure for a working vehicle is disclosed in Japanese Patent Publication Kokai No. 4-39464. According to this structure, in response to a change speed command, propelling clutches are hydraulically and automatically disengaged, actuators are driven to shift change speed gearing, and the propelling clutches are automatically engaged again.
This structure includes two main clutches acting as the propelling clutches, i.e. a first clutch (6 in FIG. 6 of the above publication) for transmitting drive from an engine to the change speed gearing, and a second clutch (19 in FIG. 6 of the publication) mounted in a drive transmitting line from the change speed gearing to drive wheels. In response to a change speed command, the second clutch is disengaged, with a working pressure for the first clutch reduced to a predetermined low level just enough to disengage the first clutch. Then, the actuators are driven to shift the change speed gearing.
Next, the second clutch is engaged quickly, and the working pressure for the first clutch is raised gradually from the predetermined low level to engage the first clutch.
By operating the first and second clutches and actuators in this way, the time consumed in one change speed operation is shortened while diminishing a shock occurring with this operation (a shock accompanying engagement of the first clutch).
However, the above structure includes a gear shift type backward-forward changeover device disposed downstream of the first clutch, change speed gearing and second clutch. With the two main clutches as well as the change speed gearing mounted in one drive transmitting line, the transmission system as a whole has a complicated structure and is costly. Thus, them is room for improvement.